Ignition capsule, which can be inductively activated, for occupant restraint systems, and a test circuit for said ignition capsule

ABSTRACT

The invention relates to a firing capsule for the gas generator of a passenger restraint system in a motor vehicle, especially for the gas generator of an inflatable impact protection cushion (airbag), in which the electrical energy necessary for detonating the igniting agent is inductively coupled in by means of alternating magnetic fields. The invention moreover relates to a test circuit with which the serviceability of the inductively activatable firing capsule can be monitored.

[0001] The invention relates to a firing capsule for the gas generatorof a passenger restraint system in a motor vehicle with an ignitingagent which is caused to explode by a resistance wire, the electricpower necessary for detonation being inductively coupled into the firingcapsule. The invention moreover relates to a test circuit by which itcan be ascertained whether the firing capsule is serviceable.

[0002] Among the most frequently used passenger restraint systems inmotor vehicles are inflatable impact protection cushions (airbags) andbelt tighteners. Both restraint systems require a gas generator with apropellant charge which is caused to explode by a firing capsule in caseof a motor vehicle collision. The amount of gas released by thisinflates the airbag or causes the turbine wheel of the belt tightener torotate.

[0003] A gas generator for an airbag is disclosed for example in U.S.Pat. No. 5,803,494 A. The known gas generator has several electricallyactivatable firing capsules, as are described for example in DE 197 33353 C1. One such firing capsule has a pot-shaped housing which is filledat least partially with an igniting agent and which is sealed by aninsulating body. Two contact pins project through the insulating bodyinto the interior of the firing capsule. The two contact pins areelectrically connected conductively to one another by a resistance wirewhich penetrates the igniting agent. In a motor vehicle collision animpact sensor is activated and sends a signal to the trigger circuit.The trigger circuit thereupon delivers a current pulse to the contactpins of the firing capsule so that the resistance wire is caused to glowor melt. The thermal energy released in doing so causes the ignitingagent to explode. The firing capsule splits at scored points intendedfor this purpose and thus allows the hot explosion gas to flow into theinterior of the gas generator. As a result the propellant charge locatedthere is caused to detonate.

[0004] One serious disadvantage of electrically activatable firingcapsules is that their resistance wire can corrode and become brittleover time so that by means of special test circuits, compare in thisregard for example DE 198 14 589 A1, it must be tested at regular timeintervals whether the firing capsule is still serviceable. The testpulses delivered by one such test circuit could however lead tomisfiring with fatal consequences under unfavorable conditions, if forexample at high speed the driver is blinded for seconds by the suddenlyinflating airbag. But misfiring can also occur due to electrostaticcharges in the motor vehicle and by leakage currents if they are routedvia the contact pins into the interior of the firing capsule.

[0005] DE 38 38 896 A1 discloses a firing capsule by which theaforementioned problems can be prevented in that the igniting agent isnot caused to explode via the heat generation of an electrically heatedresistance wire, but by incident laser light radiation. For this purposethe firing capsule is sealed with a window which is transparent to laserlight. A layer which is in thermal contact with the igniting agent andwhich is highly absorptive at the wavelength of the incident laser lightis applied to the inside of this window. Thus the layer is heated upwhen light is incident and the igniting agent is detonated. With respectto better light coupling, in the known firing capsules it is providedthat the transparent window be made as a lens. The disadvantage of theoptically activatable firing capsule is that considerable lightintensities must be made available if reliable ignition is to beensured.

[0006] The object of the invention is to devise a firing capsule for thegas generator of a passenger restraint system which essentially does notage, in which misfires as a result of electrostatic charges and leakagecurrents are precluded, and which can be detonated extremely reliably.This firing capsule is designed moreover to replace existingelectrically or optically activatable firing capsules and accordingly tobe composed such that conventional gas generators can be easilyretrofitted with it. The object of the invention is furthermore todevise a test circuit by which the serviceability of the firing capsulecan be monitored and which cannot trigger misfires.

[0007] The aspects of these two objects are achieved by a firing capsuleas claimed in claim 1 and a test circuit as claimed in claim 13;advantageous developments and embodiments are given in the dependentclaims.

[0008] The invention proceeds from the idea of inductively coupling theheat output necessary to detonate the igniting agent into the firingcapsule. As a result no electrical supply lines are necessary whichpenetrate the housing of the firing capsule so that electrostaticcharges or leakage currents which can trigger a misfire cannot be routedinto the interior of the firing capsule. The housing of the firingcapsule can be hermetically sealed because these feed lines are absent.This ensures that moisture cannot reach the resistance wire and theigniting agent. The resistance wire is thus protected against corrosionand the igniting agent remains dry and thus ignitable even under theunfavorable climatic conditions to which the motor vehicle is exposed.Moreover, the resistance wire is largely protected against temperaturefluctuations which otherwise are relayed via electrical feed lines intothe housing of the firing capsule. Accordingly it is guaranteed that theresistance wire essentially does not age or become brittle.

[0009] The firing capsule as claimed in the invention moreover enablesthe use of a test circuit which inductively couples a very weakalternating current signal into the firing capsule. In this way it ispossible with the most simple circuity means to monitor theserviceability of the firing capsule.

[0010] The principle of inductive coupling of electrical heat outputinto a consumer is known. Thus, for example U.S. Pat. No. 5,905,343 Adescribes an electrical filament lamp with a filament wire which is madeas a closed conductor loop. In the vicinity of the lamp bulb there aremagnet cores via which the alternating magnetic fields generated by theprimary windings are routed to the filament wire. These fields penetratethe secondary winding formed by the filament wire and induce in it theheating current necessary for illumination.

[0011] Furthermore, the prior art discloses firing capsules which can beinductively activated or tested. Thus DE 24 33 555 A1 disclosesproximity firing pulse transmission for airbags in which the electricalenergy needed to trigger the airbag is transmitted inductively fromstationary to rotary motor vehicle parts. The transformer used here hasa primary winding which is permanently connected to the steering column,and a secondary winding which is attached to the steering shaft or thesteering wheel and which turns coaxially in the primary winding.

[0012] DE 195 30 586 A1 discloses an arrangement for monitoring theresistance of a load which is connected to a transformer, in which anadditional resistance is connected parallel to the load and in whichthere are means which measure the primary-side input resistance of thetransformer and signal a fault of the load when the input resistancedeviates from a given value. The known arrangement is used preferablyfor monitoring the serviceability of the firing pellets of airbagsystems.

[0013] The invention is detailed below using embodiments.

[0014] FIG. 1 schematically shows a section through the firing capsuleas claimed in the invention, and in the form of a block diagram thepertinent trigger circuit and the test circuit as claimed in theinvention. The firing capsule 1 has a pot-shaped housing 2 which isfilled at least partially with an igniting agent 4. The top of thefiring capsule 1 facing the interior of the gas generator which is notshown here is made as a bursting disk 3 or has scoring so that it isensured that in the case of a motor vehicle collision the hot explosiongases travel quickly and reliably into the interior of the gasgenerator, where they cause the propellant charge to detonate. Thehousing 2 on its bottom is hermetically sealed by a sealing body 5. Inthis embodiment the sealing body 5 is penetrated by a magnet core 11. Inone preferred embodiment this magnet core 11 can also be made in twoparts so that an opening in the housing 2 is not necessary. Within thehousing 2 is a suitably held resistance wire 8 which penetrates theigniting agent 4 and which together with the secondary winding 7 forms asecondary circuit 6. What is important here is that the electricalsupply lines which are routed from the outside into the firing capsuleand thus which can be the cause of misfiring are absent. Outside thehousing 2 the magnet core 11 is surrounded by a primary winding 10. Theratio of the number of turns of the primary winding 10 to the secondarywinding 7 is such that in the secondary circuit 6 a relatively strongcurrent is induced which reliably causes the resistance wire 8 to glowor melt and thus causes the explosion of the igniting agent 4.

[0015] The primary winding 10 together with a trigger circuit 12 forms aprimary circuit 9. The trigger circuit 12 is connected to an impactsensor 13. In the case of motor vehicle collision the impact sensor 13releases a signal to the trigger circuit 12, as a result of which itgenerates a relatively large AC voltage. The AC voltage can be forexample sinusoidal or rectangular; what is decisive for the operation ofthe firing capsule is simply that as a result of this AC voltage on thesecondary side an alternating current is induced which heats theresistance wire 8 enough.

[0016] A test circuit 14 is connected parallel to the primary winding10. At regular time intervals, for example at an interval of a fewseconds, or continuously, this circuit produces a weak AC voltage with afrequency which can be clearly different from the AC voltage which ismade available for detonation, therefore for example can be much higher.As long as the resistance wire 8 together with the secondary winding 7forms a closed secondary circuit 6, this circuit is therefore in otherwords intact, the test circuit 14, as soon as it delivers its testsignals to the primary winding 10, is loaded with respect to power in anexactly defined manner. If at this point for example the resistance wire8 breaks, which can be the result of continuous vehicle vibrations, thesecondary circuit 6 is suddenly open. The load on the test circuit 14changes as a result. To implement the test circuit 14 as claimed in theinvention, besides an AC voltage generator for generating the testvoltage, therefore only one circuit is necessary which detects a changein the load by the secondary circuit and thereupon delivers a warningsignal, which can be for example acoustic or optical. A circuit in whicha test signal is inductively coupled into the current feed line of aconventional firing capsule of an airbag is known from publication DE 3812 633 C2. In contrast to this prior art, the test signal in the testcircuit as claimed in the invention is coupled into a secondary circuitwhich is completely electrically insulated from the environment andwhich contains the resistance wire 8. One special advantage of thisarrangement is that the test signal, if it causes only minimum heatingof the resistance wire, can also be sent quite without interruption.

[0017] In the embodiment shown in FIG. 2, the secondary circuit 6consists only of one closed ring of resistance wire 8, therefore of asecondary winding with exactly one turn. On either side of the firingcapsule there are a first primary winding 10 a and a second primarywinding 10 b with one magnet core 11 a, 11 b each. The two primarywindings 10 a, 10 b are supplied with alternating current in case of amotor vehicle collision by two trigger circuit 12 a, 12 b in the aboveexplained manner. The magnetic fields produced in doing so penetrate theclosed ring of resistance wire in the same direction, as symbolized inFIG. 2 by an arrow, and induce the heating current necessary fordetonating the igniting agent 4.

[0018] This special configuration of the firing capsule as claimed inthe invention enables very effective protection against misfiring causedby faults in the trigger circuit. If the alternating currents flowing inthe two primary circuits 9 a, 9 b are dimensioned such that first thesimultaneous interaction of the two heats up the resistance wire 8 tothe required detonation temperature, then one of the primary circuitscan be actuated as a result of a technical fault, without misfiring withthe above described fatal consequences occurring. Of course the firingcapsule as claimed in the invention can also be equipped with more thantwo interacting primary circuits. In the preferred embodiment there are4 primary windings around the firing capsule, with an inductive powerdimensioned such that only the interaction of 3 primary circuits leadsto detonation. Therefore the system also remains fully serviceable evenif one of the primary circuits has failed. At the same time misfires ofthe aforementioned type are reliably prevented. Of course at least oneof the primary circuits can again be equipped with a test circuit asclaimed in the invention.

[0019] FIG. 3a shows in a perspective how the resistance wire which ismade as a closed conductor loop can be safely accommodated in the firingcapsule 1 as claimed in the invention. For this purpose the resistancewire 8 is fixed on a carrier plate 15 of insulating material. Here thearrow also illustrates how the secondary circuit is penetrated by thealternating magnetic field of the primary circuit for purposes ofinductive coupling. It is advantageous if the wire, as is shown in FIG.3b in a section, is inserted into a groove 16 of the carrier plate 15.Reference number 4 in turn labels the igniting agent. FIG. 3c shows in asection another embodiment in which two annular resistance wires 8 a, 8b are mounted electrically insulated from one another on either side ofthe carrier plate 15. If one of the resistance wires fails for exampledue to rupture or corrosion, the firing capsule still remains fullyserviceable. It goes without saying that several arrangements of thetype shown in FIG. 3c can be housed stacked on top of one another ornext to one another in a firing capsule as claimed in the invention. Inthis case it is especially advantageous if the firing capsule 1 issurrounded by at least one primary winding 10; 10 a, 10 b and forexample is used as the winding body for the primary winding. Anadditional magnet core for the primary winding is not necessary in thisembodiment.

[0020] The object of the invention is furthermore to be able to producethe secondary circuit which is responsible for detonation in a manner assimple and economical as possible.

[0021] This further aspect of the object is achieved by an inductivelyactivatable firing capsule with the features listed in claim 16.

[0022] The basic idea is to form at least one secondary circuit which islocated within the firing capsule housing by a closed resistanceelement. This resistance element can be produced in diverse ways, forexample by coating an annular core of insulating or poorly conductivematerial with a suitably dimensioned metal layer. Alternatively acarrier plate can be used which has an annular groove which is filledwith metal. Likewise the metal can be applied by conventional coatingprocesses such as for example by vapor deposition directly to thecarrier plate.

[0023] The invention is detailed below using some embodiments shown inthe drawings.

[0024] FIG. 4 shows a section through the firing capsule as claimed inthe invention which is labeled 1 overall. The firing capsule 1 has afiring capsule housing 2 which is hermetically sealed by a sealing body5. In contrast to conventional firing capsules, in this embodimentcurrent supply wires which penetrate the sealing body 5 are notnecessary in this version. The firing capsule 1 is dimensioned andconfigured overall such that conventional passenger restraint systemscan be easily equipped with it. Within the firing capsule housing 2 is ahigh- explosive igniting agent 4 which in the case of a motor vehiclecollision is caused to explode by the heat generation of a resistanceelement. For this reason, in the sealing body 5 there is a closed groove16 into which at least one metal layer 17 is placed. The heat outputrequired for detonation is inductively coupled into the firing capsule.For this reason, outside of the firing capsule there are two primarywindings 10 a, 10 b with a host of turns which are wound onto a commoncore 11, preferably of magnetizable material, for better routing of themagnetic flux. In the case of a motor vehicle collision an impact sensor13 delivers a signal which is illustrated by an arrow to two triggercircuits 9 a, 9 b, of which each then sends a trigger current into theprimary winding 10 a and 10 b assigned to it.

[0025] The entire arrangement is configured such that the magneticfields produced in the two primary windings 10 a, 10 b are rectified ateach instant, therefore overall yield a magnetic field which, as thevertical arrow is designed to show, penetrates the secondary circuitfree of loss as much as possible. The two primary circuits, as havealready been discussed in detail, are dimensioned for reasons of safetysuch that the heat output which can be inductively coupled into thesecondary circuit by each individual one of them, is not enough to causethe igniting agent 4 to explode.

[0026] Misfires are thus essentially precluded. Only when the twoprimary circuits are activated by the signal of the impact sensor 13does detonation occur. It can be especially advantageous to trigger eachof the primary circuits via a separate impact sensor 13. It furthermoregoes without saying that more than two primary circuits can also beused; in a simpler version the novel firing capsule of course makes dowith a single primary circuit.

[0027] In the embodiment shown in FIG. 5, the firing capsule as claimedin the invention is not sealed by a separate sealing body 5, but has analready closed firing capsule housing 2 in which the igniting agent 4 islocated. Within the firing capsule housing 2 there is a carrier plate 15which is surrounded by the igniting agent 4 and which on each side hasan annular groove 16 in which at least one metal layer 17 is placed. Inthis embodiment there are thus two secondary circuits which areindependent of one another. Here the vertical arrow also indicates themagnetic flux which is produced by at least one primary winding which isno longer shown here, in the case of a motor vehicle collision. In thetwo resistance elements this magnetic flux induces currents by which thetwo are heated up and thus cause the igniting agent 4 to explode. Thefiring capsule as claimed in the invention then also remains serviceableeven if one of the two secondary circuits for example should fail due toa break in the resistance element.

[0028] FIGS. 6a to 6e show as a section five possibilities forimplementing the resistance elements for the inductively activatablefiring capsule as claimed in the invention.

[0029] In the embodiment as shown in FIG. 6a a closed, annular groovewhich is filled with at least one metal layer 17 is machined into acarrier plate 15. This corresponds essentially to the arrangement asshown in FIG. 4, but with the difference that there is only one singleresistance element. In the embodiment as shown in FIG. 6b at least onemetal layer 17 surrounds the carrier plate 15 on its outside edge like atire. FIG. 6c shows an alternative in which at least one metal layer 17is applied directly to the surface of the carrier plate 15. Variousprocesses are possible for this production step, for example vapordeposition using the corresponding masks or large-area coating withsubsequent etching out of the desired conductor structure. In theembodiment as shown in FIG. 6d the carrier plate 15 was first completelycoated with at least one metal layer 17 and then provided with a hole sothat only the annular, closed resistance element remains. Finally, FIG.6e shows one especially preferred version in which an annular core 18 ofinsulating or poorly conductive material has been coated with at leastone metal layer.

1. Inductively activatable firing capsule for the gas generator of a passenger restraint system in a motor vehicle, especially for a gas generator of an inflatable impact protection cushion, with a pot-shaped housing (2) which is filled at least partially with an igniting agent (4), outside the housing (2) there being at least one primary circuit (9; 9 a, 9 b) with a primary winding (10; 10 a, 10 b) which in the case of a vehicle collision is supplied with a trigger current, and within the housing (2) there being at least one secondary circuit (6) which contains a secondary winding (7) and connected in series thereto a resistance wire (8), the resistance wire (8) penetrating the igniting agent (4) or being located in the immediate vicinity of the igniting agent (4), and at least one secondary winding (7) being inductively coupled to at least one primary winding (10; 10 a, 10 b).
 2. Firing capsule as claimed in claim 1, wherein the housing (2) is hermetically sealed by a sealing body (5).
 3. Firing capsule as claimed in claim 1, wherein the housing (2) consists of an insulating material.
 4. Firing capsule as claimed in claim 2, wherein the sealing body (5) consists of an insulating material.
 5. Firing capsule as claimed in claim 1, wherein at least one primary winding (10; 10 a, 10 b) and at least one secondary winding (7) each surround a magnet core (11; 11 a, 11 b).
 6. Firing capsule as claimed in claim 5, wherein there is a common magnet core (11) which penetrates the sealing body (5).
 7. Firing capsule as claimed in claim 5, wherein at least one primary winding (10; 10 a, 10 b) and at least one secondary winding (7) each have its own magnet core (11; 11 a, 11 b).
 8. Firing capsule as claimed in claim 1, wherein at least one secondary circuit (6) consists of an annularly closed resistance wire (8) which is located in the housing (2) of the firing capsule (1) such that it is inductively coupled to at least one primary winding (10; 10 a, 10 b) outside of the housing (2).
 9. Firing capsule as claimed in claim 8, wherein there is at least one annularly closed resistance wire (8) on the carrier plate (15).
 10. Firing capsule as claimed in claim 9, wherein two annularly closed resistance wires (8; 8 a, 8 b) at a time are located electrically insulated from one another on opposite sides of each carrier plate (15).
 11. Firing capsule as claimed in claim 1, wherein there are at least two primary circuits (9; 9 a, 9 b) and one secondary circuit (6), the electrical power which is coupled into the secondary circuit (6) by one of the primary circuits (9; 9 a, 9 b) at a time being dimensioned such that the igniting agent (4) is not caused to explode.
 12. Firing capsule as claimed in claim 1, wherein the trigger current is a sinusoidal or rectangular alternating current.
 13. Test circuit for an inductively activatable firing capsule as claimed in one of claims 1 to 12, wherein it is connected in parallel to at least one primary winding (10; 10 a, 10 b) and delivers a weak AC voltage signal to at least one primary winding (10; 10 a, 10 b) and wherein it contains a device which detects the change of the alternating current flowing in at least one primary winding (10; 10 a, 10 b).
 14. Test circuit as claimed in claim 13, wherein it produces an alarm signal when the alternating current flowing in at least one primary winding (10; 10 a, 10 b) changes.
 15. Test circuit as claimed in claim 13, wherein the weak AC voltage signal is delivered at regular time intervals or continuously.
 16. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein at least one secondary circuit located within the firing capsule housing (2) is formed by a closed resistance element.
 17. Inductively activatable firing capsule as claimed in claim 16, wherein the closed resistance element has a rectangular, square or round cross section.
 18. Inductively activatable firing capsule as claimed in claim 16, wherein the closed resistance element has an annular configuration.
 19. Inductively activatable firing capsule as claimed in one of claims 16 to 18, wherein the closed resistance element has fewer than 10, preferably exactly one turn.
 20. Inductively activatable firing capsule as claimed in one of claims 16 to 19, wherein the closed resistance element is formed by an annular core (18) of insulating or poorly conductive material which is coated by at least one metal layer (17).
 21. Inductively activatable firing capsule as claimed in one of claims 16 to 19, wherein the closed resistance element consists of at least one metal layer (17) which is applied to a carrier plate (15) of insulating or poorly conductive material.
 22. Inductively activatable firing capsule as claimed in one of claims 16 to 19, wherein the closed resistance element consists of at least one metal layer (17) which is embedded in a groove (16) of a carrier plate (15) of insulating or poorly conductive material.
 23. Inductively activatable firing capsule as claimed in claim 21 or 22, wherein the carrier plate (15) is formed by the sealing body (5) of the firing capsule housing (2).
 24. Inductively activatable firing capsule as claimed in claim 20, wherein the annular core (18) consists of glass, ceramic or plastic.
 25. Inductively activatable firing capsule as claimed in one of claims 21 to 23, wherein the carrier plate (15) consists of glass, ceramic or plastic.
 26. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein at least one metal layer (17) consists of metals with a high specific resistance.
 27. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein there are at least two primary circuits which are triggered by a common impact sensor (13).
 28. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein there are at least two primary circuits, of which each is triggered by a common impact sensor (13). 